Something that separates.
One example is religion, another is music. My favorite is mountain ranges. Something that has many parameters that can be varied to a large degree will often divide people and things. Is this bad? Depends on your perspective.
Without divisions there would be no competition, and there would be no change. Competition comes about because of the small diffenences that divide us. Some divisions of society will be successful and should think separation is good. Others will not and they should think separation is bad. Oddly, what they really think is the opposite of this.